1. Field of the Invention
This invention relates to an abnormality detection system for detecting abnormalities in evaporative fuel control systems of internal combustion engines which temporarily store evaporative fuel generated in the fuel tank in a canister thereof and discharge same into the intake system of the engine.
2. Prior Art
The temperature inside of a canister of an evaporative fuel control system rises as evaporative fuel is stored into the canister (hereinafter called "charging"), due to heat of adsorption, while the former lowers as the latter is discharged into the intake system (hereinafter called "purging"). There has been proposed an abnormality detecting system for evaporative fuel control systems, which makes use of the above fact, by U.S. Pat. No. 4,949,695. This proposed system includes a temperature sensor arranged in the canister, and calculates the difference (temperature drop amount) between the maximum temperature inside the canister assumed during stoppage of the vehicle when charging is effected and the minimum temperature inside the canister assumed during running of the vehicle when purging is effected, and compares the calculated difference with a predetermined value, to determine whether there is an abnormality in the evaporative fuel control system, based upon the result of the comparison.
According to the proposed system, the temperature inside the canister is detected at a single point. However, the location within the canister at which the temperature changes with charging, varies depending upon the charged amount. As a result, the charging state of the canister cannot be accurately grasped such that abnormality detection cannot be accurately effected. Particularly, when evaporative fuel is charged into the canister at a small flow rate or the concentration of evaporated fuel charged is low, it cannot be discriminated whether a change in the detected temperature inside the canister is caused by a change in the ambient temperature or due to heat of adsorption of evaporative fuel, which makes it impossible to make an accurate detection of abnormality.
Further, the proposed system makes it a prerequisite that charging should be effected during stoppage of the vehicle and purging during running of the vehicle, to carry out an abnormality detection depending upon the result of comparison of the temperature difference with the predetermined value. Therefore, it cannot start the abnormality detection until after the vehicle has run over a time period (e.g. 30 minutes) elapses, which is required for the temperature inside the canister to lower to a sufficient extent if purging is positively effected while no abnormality exists. Thus, it takes a long time to obtain a result of the abnormality detection.
Still further, during stoppage of the vehicle, the amount of evaporative fuel charged into the canister largely varies depending on the fuel amount, pressure, and temperature within the fuel tank. Therefore, if an abnormality detection is carried out by starting purging when the charged amount within the canister does not reach yet a sufficient level, there is a possibility of an erroneous judgement that an abnormality exists even when the evaporative fuel control system is normally functioning.
On the other hand, there is widely known an air-fuel ratio control system which determines an air-fuel ratio correction coefficient in response to an air-fuel ratio signal from an air-fuel ratio sensor arranged in the exhaust system of an internal combustion engine, and controls the amount of fuel supplied to the engine by the use of the determined correction coefficient. If evaporative fuel is purged into the intake system during operation of the air-fuel ratio control system, the value of the air-fuel ratio correction coefficient changes due to enriching of the air-fuel ratio of a mixture supplied to the engine. An abnormality detecting method for detecting an abnormality in the evaporative fuel control system has been proposed, which makes use of the above fact, by Japanese Patent Application No. 2-207914 filed by the assignee of the present application. According to this proposed method, after the engine has been warmed up when the engine temperature exceeds a predetermined value, the value of the air-fuel ratio correction coefficient is determined as a first value thereof during stoppage of purging, and also determined as a second value thereof during purging, and the first and second values are compared with each other. An abnormality detection is effected based upon the comparison result.
The proposed method makes it a precondition that the canister is fully charged with evaporative fuel (evaporative fuel is fully adsorbed within the canister) immediately before purging is started. More specifically, in this method, the above precondition is assumed to be satisfied when the engine and the vehicle on which the engine is installed have continued a steady operating condition and a steady running condition, respectively, over a predetermined time period after completion of warming-up of the engine, and then the second value of the air-fuel ratio correction coefficient is determined to effect an abnormality detection.
However, for example, when evaporative fuel is not fully generated within the fuel tank, the canister is not fully charged even after the steady conditions have continued over the predetermined time period. If an abnormality detection is effected on such an occasion, there is a possibility of an erroneous judgement that there is an abnormality in the evaporative fuel control system, though no fault exists in actuality.